X-Virus-Scanned: clean according to Sophos on Logan.com Return-Path: Sender: To: lml Date: Wed, 25 Nov 2009 08:17:46 -0500 Message-ID: X-Original-Return-Path: Received: from nskntmtas01p.mx.bigpond.com ([61.9.168.137] verified) by logan.com (CommuniGate Pro SMTP 5.3c2) with ESMTP id 3987686 for lml@lancaironline.net; Tue, 24 Nov 2009 06:47:53 -0500 Received-SPF: pass receiver=logan.com; client-ip=61.9.168.137; envelope-from=frederickmoreno@bigpond.com Received: from nskntotgx03p.mx.bigpond.com ([58.170.181.250]) by nskntmtas01p.mx.bigpond.com with ESMTP id <20091124114714.JXUQ10503.nskntmtas01p.mx.bigpond.com@nskntotgx03p.mx.bigpond.com> for ; Tue, 24 Nov 2009 11:47:14 +0000 Received: from Razzle ([58.170.181.250]) by nskntotgx03p.mx.bigpond.com with ESMTP id <20091124114711.HDDE24930.nskntotgx03p.mx.bigpond.com@Razzle> for ; Tue, 24 Nov 2009 11:47:11 +0000 From: "Frederick Moreno" X-Original-To: "Lancair Mail" Subject: FW: Fuel Pressure question X-Original-Date: Tue, 24 Nov 2009 20:47:08 +0900 X-Original-Message-ID: <00d101ca6cfb$dc12a140$9437e3c0$@com> MIME-Version: 1.0 Content-Type: multipart/alternative; boundary="----=_NextPart_000_00D2_01CA6D47.4BFA4940" X-Mailer: Microsoft Office Outlook 12.0 Thread-Index: AcpsX4RAoS6fFsGATbyMbJoK3onovwAmf8FA Content-Language: en-au X-RPD-ScanID: Class unknown; VirusThreatLevel unknown, RefID str=0001.0A150205.4B0BC7C2.00BF,ss=1,fgs=0 X-SIH-MSG-ID: rBw0GdX8TFa2kTAvmTy2alorgFm6/gF5uMhSBI0wt0lHEVbGsd/fRMq9cqtA34zj1C4oc0+VQD5mc67gTY7QuN23K71VSrLZ5/E2 This is a multi-part message in MIME format. ------=_NextPart_000_00D2_01CA6D47.4BFA4940 Content-Type: text/plain; charset="us-ascii" Content-Transfer-Encoding: 7bit "The unmetered side of the fuel metering unit or the metered side? and why. And lastly, where have you mounted the fuel pressure transducer? EI suggests the firewall, but that leads to a long fuel line and ? does that make any difference in the readings, dampening etc. Can you mount it closer or are the vibrations going to cause failure of the fitting and fuel leaking over hot cylinders?" I have an MVP 50 engine monitor with fuel pressure sensor mounted on the firewall and reading unmetered fuel pressure. It does lead to a long line from throttle to firewall (and it must be fire shielded), but does not seem to cause difficulties. I did have a sensor go erratic at about 100 hours and replaced it with a new one. I suspect vibration in the wore harness caused the failure, and the new installation has better strain and vibration relief on the lines going to the pressure sensor. The installation was exactly as outlined in the installation manual. The sensor installation on the engine presumably causes problems due to vibration on the engine which eventually affects the sensor which probably has a tiny diaphragm and strain gage attached as part of a measurement bridge since it is a four wire connection with ground and 5 volts on one pair of wires, and the others sending the signal to the black box. I plotted unmetered fuel pressure (before the metering valve on the throttle body) vs. power setting and found that as you retard the engine down toward idle (very low power settings as might occur on descent), the fuel pressure jumps up as you described. I also got high pressure alarms with higher RPM and lower power settings such as slowing on downwind to get the gear down. I believe what is happening is that the fuel metering valve connected to the throttle shaft has some complex, precision passages machined into it to match the fuel flow to the air flow as the throttle is closed. Recall that the air flow past a butterfly valve (throttle valve) is highly non-linear. The first little bit of opening causes large increases in air flow (compared to idle position) and progressively moving the throttle to fully open results in less and less additional air flow. Thus to match this rapid change in air flow when opening off of idle position the fuel valve has to have some fancy passages that open rapidly as well as the shaft rotates. Conversely, as you approach the idle position of the throttle, the fuel valve is going to start aggressively cutting off the fuel flow. I believe this is what we have seen and you are seeing. As you approach the idle position of the throttle, but with higher RPM, the positive displacement fuel pump starts generating higher unmetered fuel pressures as the fuel valve closes. Momentary high pressure in flight with throttle closure would be expected, it would seem to me, and that is exactly what happens. Why not measure metered fuel pressure instead? Beats me. Seems like it would be more useful as it would correlate with flow rate. However, I suspect that at lower fuel flows, the fuel pressure is essentially zippo and the fuel is dribbling through the injectors and being atomized by the high engine vacuum (low manifold pressure) that prevails when the idle is near to idle. Engine specs for fuel pressure are also listed in terms of unmetered fuel pressure. Clear as mud? Caveat: While I have studied the Continental fuel injection manuals and procedures bulletins carefully, I am no expert. Fred Moreno ------=_NextPart_000_00D2_01CA6D47.4BFA4940 Content-Type: text/html; charset="us-ascii" Content-Transfer-Encoding: quoted-printable

“The unmete= red side of the fuel metering unit or the metered side?  and why.    = And lastly, where have you mounted the fuel pressure transducer?  EI sugge= sts the firewall, but that leads to a long fuel line and ? does that make any difference in the readings, dampening etc.  Can you mount it closer or= are the vibrations going to cause failure of the fitting and fuel leaking over = hot cylinders?”

I have an MVP 50 engine monitor with fuel pre= ssure sensor mounted on the firewall and reading unmetered fuel pressure.  I= t does lead to a long line from throttle to firewall (and it must be fire shielded), but does not seem to cause difficulties.  I did have a sens= or go erratic at about 100 hours and replaced it with a new one.  I suspe= ct vibration in the wore harness caused the failure, and the new installation = has better strain and vibration relief on the lines going to the pressure sensor.  The installation was exactly as outlined in the installation = manual.  The sensor installation on the engine presumably causes problems due to vibration on the engine which eventually affects the sensor which probably = has a tiny diaphragm and strain gage attached as part of a measurement bridge s= ince it is a four wire connection with ground and 5 volts on one pair of wires, = and the others sending the signal to the black box.

 

I  plotted unmetered fuel pressure (before the metering valve on the throttle body) vs. power setting and found that as you retard = the engine down toward idle (very low power settings as might occur on descent)= , the fuel pressure jumps up as you described.  I also got high pressure alarms with higher RPM and lower power settings such as slowing on downwind= to get the gear down. 

 

I believe what is happening is that the fuel metering valve connected to the throttle shaft has some complex, precision passages machin= ed into it to match the fuel flow to the air flow as the throttle is closed.&n= bsp; Recall that the air flow past a butterfly valve (throttle valve) is highly non-linear.  The first little bit of opening causes large increases in= air flow (compared to idle position) and progressively moving the throttle to&n= bsp; fully open results in less and less additional air flow. =

 

Thus to match this rapid change in air flow when opening off= of idle position the fuel valve has to have some fancy passages that open rapi= dly as well as the shaft rotates.  Conversely, as you approach the idle position of the throttle, the fuel valve is going to start aggressively cut= ting off the  fuel flow.

 

I believe this is what we have seen and you are seeing. = ; As you approach the idle position of the throttle, but with higher RPM, the positive displacement fuel pump starts generating higher unmetered fuel pressures as the fuel valve closes.  Momentary high pressure in flight with  throttle closure would be expected, it would seem to me, and tha= t is exactly what  happens. 

 

Why not measure metered fuel pressure instead?  Beats me.  Seems like it would be more useful as it would correlate with flo= w rate.  However, I suspect that at lower fuel flows, the fuel pressure is essential= ly zippo and the fuel is dribbling through the injectors and being atomized by= the high engine vacuum (low manifold pressure) that prevails when the idle is n= ear to idle.   Engine specs for fuel pressure are also listed in term= s of  unmetered fuel pressure.

 

Clear as mud?

 

Caveat: While I have studied the Continental fuel injection = manuals and procedures bulletins carefully, I am no expert.

 

Fred Moreno

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